According to the recent demand for cars having a low fuel consumption ratio, a conjugated diene-based polymer having modulational stability represented by wet skid resistance as well as low rolling resistance, and excellent abrasion resistance and tensile properties is required as a rubber material for a tire.
In order to reduce the rolling resistance of a tire, there is a method of reducing hysteresis loss of a vulcanized rubber, and rebound resilience at 50° C. to 80° C., tan δ, or the like is used as an evaluation index of the vulcanized rubber. That is, it is desirable to use a rubber material having high rebound resilience at the above temperature or a low tan δ value.
Natural rubber, polyisoprene rubber, or polybutadiene rubber is known as a rubber material having low hysteresis loss, but these rubbers have a limitation of low wet skid resistance. Thus, recently, a conjugated diene-based (co)polymer, such as styrene-butadiene rubber (hereinafter, referred to as “SBR”) and butadiene rubber (hereinafter, referred to as “BR”), is prepared by emulsion polymerization or solution polymerization to be used as rubber for a tire. Among these polymerization methods, the greatest advantage of the solution polymerization in comparison to the emulsion polymerization is that the vinyl structure content and the styrene content, which specify physical properties of the rubber, may be arbitrarily adjusted and its molecular weight and physical properties may be controlled by coupling or modification. Thus, the SBR prepared by the solution polymerization is widely used as a rubber material for a tire because it is easy to change a structure of the finally prepared SBR or BR, and movement of chain terminals may be reduced and a coupling force with a filler, such as silica and carbon black, may be increased by coupling or modification of the chain terminals.
In a case where the solution-polymerized SBR is used as the rubber material for a tire, since a glass transition temperature of the rubber is increased by increasing the vinyl content in the SBR, physical properties, such as running resistance and braking force, required for a tire may not only be controlled, but fuel consumption may also be reduced by appropriately adjusting the glass transition temperature.
The solution-polymerized SBR is prepared by using an anionic polymerization initiator, and is being used by coupling or modification of chain terminals of the polymer thus formed using various modifiers.
For example, U.S. Pat. No. 4,397,994 discloses a method of coupling active anions of the chain terminals of a polymer obtained by polymerizing styrene-butadiene using alkyllithium which is a monofunctional initiator in a non-polar solvent, using a binder such as a tin compound.
Meanwhile, carbon black and silica are being used as a reinforcing filler of a tire tread, wherein, in a case where the silica is used as the reinforcing filler, the hysteresis loss may be low and the wet skid resistance may be improved. However, since the silica having a hydrophilic surface has a low affinity with the rubber in comparison to the carbon black having a hydrophobic surface, dispersibility may be poor, and, thus, there is a need to use a separate silane coupling agent to improve the dispersibility or provide coupling between the silica and the rubber.
Therefore, a method of introducing a functional group having an affinity or reactivity with the silica into the terminals of rubber molecules is being performed, but its effect is insufficient.
Accordingly, the development of rubber having high affinity with a filler such as silica is required.